Method of producing a segmented abradable ceramic coating system

ABSTRACT

A method of producing a segmented abradable ceramic coating system having superior abradability and erosion resistance is disclosed. The system includes a duct segment having a metallic substrate, a MCrAlY bond coat on the substrate and a segmented abradable ceramic coating on the bond coat. The segmented abradable ceramic coating includes a base coat foundation layer, a graded interlayer and an abradable top layer for an overall thickness of preferably about 50 mils (1.270 mm). The coating is characterized by a plurality of vertical microcracks. By precisely controlling the deposition parameters, composition of the layers and layer particle morphology, segmentation is achieved, as well as superior abradability and erosion resistance.

This is a division of copending application Ser. No. 08/534,146 filed onSep. 26, 1995 pending.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to duct segments for use in gas turbineengines, and particularly to ceramic abradable coatings for such ductsegments.

2. Background Information

Modern gas turbine engines, particularly those used in aircraft, operateat high rotational speeds and high temperatures for increasedperformance and efficiency. The turbine of a mode gas turbine engine istypically of an axial flow design and includes a plurality of axial flowstages. Each axial flow stage comprises a plurality of blades mountedradially at the periphery of a disk which is secured to a shaft. Aplurality of duct segments surround the stages to limit the leakage ofgas flow around the tips of the blades. These duct segments are locatedon the inner surface of a static housing or casing. The incorporation ofthe duct segments improves thermal efficiency because more work may beextracted from gas flowing through the stages as opposed to leakingaround the blade tips.

Although the duct segments limit the leakage of gas flow around theblade tips, they do not completely eliminate the leakage. It has beenfound that even minor amounts of gas flow around the blade tipsdetrimentally affect turbine efficiency. Thus, gas turbine enginedesigners proceed to great lengths to devise effective sealingstructures. These structures generally include a coated duct segment incombination with a blade tip coating which renders the tips resistant towear. In operation, the tips provide sealing by cutting into the coatingon the duct segment.

Unfortunately current duct segment coatings, which are typicallyceramic, suffer from excessive material loss as a result of erosion orspalling. In general, erosion is the wearing away of coating materialdue to factors such as abrasion and corrosion. Erosion often resultsfrom particle impingement during engine operation. Spalling is typicallycaused by delamination cracking at the ceramic-metal interface resultingfrom thermal stress and the aggressive thermal environment. Spalling isessentially piecemeal coating loss consisting of many small coherentvolumes of coating material. Ceramic coating loss increases blade tipclearance and thus is detrimental to turbine efficiency, as well asdetrimental to the blades themselves. For example, the blades may becomedamaged due to the increased temperature at which the engine must thenoperate to make up for lost thrust.

Accordingly, there exists a need for a coating which is abradable aswell as erosion and spalling resistant. This coating is necessary for asealing system having superior abradability and erosion resistance.

DISCLOSURE OF INVENTION

The present invention is directed towards providing a coating which isabradable as well as erosion and spalling resistant.

An aspect of the invention includes a segmented abradable ceramiccoating system having enhanced abradability. The system comprises a ductsegment including a metallic substrate; a MCrAlY bond coat on thesubstrate; and a segmented abradable ceramic (SAC) coating on the MCrAlYbond coat. The nature of the MCrAlY bond coat is such that it mustprovide sufficient resistance to oxidation and corrosion. One aspect ofthe SAC coating comprises three ceramic layers which include a base coatfoundation layer of material selected from the group consisting ofzirconia stabilized with ceria, zirconia stabilized with magnesia,zirconia stabilized with calcia, zirconia stabilized with yttria, andmixtures thereof; an abradable top layer comprising zirconia; and agraded interlayer which is a compositional blend of the base coatfoundation layer and the abradable top layer. The graded interlayer ispositioned between the base coat foundation layer and the abradable toplayer. The segmented abradable ceramic coating also includes a pluralityof vertical microcracks and the three ceramic layers comprise powderparticles which are spherical and hollow (prior to deposition) forincreased abradability.

Another aspect of the invention includes a segmented abradable sealingsystem having enhanced abradability relative to segmented sealingsystems known in the art. The sealing system comprises a duct segmentincluding a metallic substrate; a MCrAlY bond coat on the substrate; anda segmented abradable ceramic coating on the MCrAlY bond coat. Thesystem also includes a cooperating interacting turbine component havingan abrasive coating on a portion thereon such that the abrasive coatingcan interact with the segmented abradable ceramic coating to providesealing.

Yet another aspect of the invention includes a method of making asegmented abradable ceramic coating. By precisely controlling thedeposition parameters and utilizing specific powder compositions andpowder morphology, segmentation of the coating into vertical microcracksis achieved as well as superior abradability and erosion resistance.

An advantage of the present invention is segmentation of the coatinginto columnar type cells thereby significantly improving ceramicspalling resistance. The novel plasma spray processing parameters of thepresent invention produce ceramic segmentation which enhances erosionresistance and results in superior abradability.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following description andaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-section of a portion of a modem gas turbine enginewith a coated flow path duct segment of the present invention positionedtherein.

FIG. 2 shows a coated flow path duct segment of the present invention indetail.

BEST MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1, a plurality of blades 2 mounted on a disk rotateabout an axis of rotation 4 in the turbine section of a gas turbineengine. A static housing, concentric with the axis of rotation 4,surrounds the blades 2. A gap 8 exists between the housing and tips 11of blades 2.

During gas turbine engine operation, high temperature gas flows betweenthe rotating turbine blades 2. The turbine must efficiently convert theenergy of this high temperature gas into shaft horsepower to drive acompressor. Thus, gas leakage through gap 8 must be minimized becauseminute changes in tip clearance have a great adverse effect on gasturbine engine performance.

Accordingly, a flow path duct segment 10 is provided between the bladetips 11 and the housing. As shown in FIG. 2, the flow path duct segment10 includes a substrate or seal shoe 12 which is made of a nickel orcobalt base superalloy and is typically fabricated by casting andmachining. The substrate 12 is located on the inner wall of the housing.

An abradable ceramic coating system is applied to the substrate 12.Prior to depositing the coating system, the substrate 12 may be cleanedto remove contamination. Cleaning is conventional and may includealuminum oxide grit blasting.

A bond coat 14 of an MCrAlY material is applied to the substrate 12.MCrAlY refers to known metal coating systems in which M denotes nickel,cobalt, iron, or mixtures thereof; Cr denotes chromium; Al denotesaluminum; and Y denotes yttrium. MCrAlY materials are often known asoverlay coatings because they are put down in a predeterminedcomposition and do not interact significantly with the substrate duringthe deposition process. For examples of MCrAlY materials see U.S. Pat.No. 3,528,861 which describes a FeCrAlY coating as does U.S. Pat. No.3,542,530. In addition, U.S. Pat. No. 3,649,225 describes a compositecoating in which a layer of chromium is applied to a substrate prior tothe deposition of a MCrAlY coating. U.S. Pat. No. 3,676,085 describes aCoCrAlY overlay coating while U.S. Pat. No. 3,754,903 describes aNiCoCrAlY overlay coating having particularly high ductility. U.S. Pat.No. 4,078,922 describes a cobalt base structural alloy which derivesimproved oxidation resistance by virtue of the presence of a combinationof hafnium and yttrium. A preferred MCrAlY bond coat composition isdescribed in U.S. Pat. No. 32,121, which is assigned to the presentAssignee and incorporated herein by reference, as having a weightpercent compositional range of 5-40 Cr, 8-35 Al, 0.1-2.0 Y, 0.1-7 Si,0.1-2.0 Hf, balance selected from the group consisting of Ni, Co andmixtures thereof. See also U.S. Pat. No. 4,585,481, which is alsoassigned to the present Assignee and incorporated herein by reference.

This MCrAlY bond coat 14 may be applied by any method capable ofproducing a dense, uniform, adherent coating of desired composition. Forexample, techniques such as sputtering, electron beam physical vapordeposition and high velocity plasma spray techniques are known. In thelatter technique, a spray torch may operate in a vacuum chamber at apressure of less than about 60 torr (60 mm Hg) or in another suitableatmosphere, such as air. If a vacuum chamber is employed, the substrateis heated to a temperature between about 1500° F. (816° C.) and about1900° F. (1038° C.). If an air atmosphere is used, the substratetemperature is maintained at less than about 600° F. (316° C.).Preferably, however, the bond coat is applied by a process known as highvelocity oxy-fuel (HVOF) spray. This deposition process utilizes a spraytorch in which fluid fuel or gas is combusted with oxygen to produce ahigh velocity gas stream into which powdered coating material isinjected, heated and propelled onto the part. This process is effectiveas well as cost efficient.

The particle size for the bond coat 14 may be between about 15 microns(0.015 mm) and about 60 microns (0.060 mm), with preferably a meanparticle size of about 25 microns (0.025 mm). The bond coat may beapplied to a thickness between about 5 mils (0.127 mm) and about 10 mils(0.254 mm). Preferably the thickness is between about 6 mils (0.152 mm)and about 7 mils (0.178 mm).

Next a segmented abradable ceramic (SAC) coating 16 is applied on bondcoat 14. The SAC coating 16 comprises three ceramic layers which areindividually applied for an overall thickness between about 20 mils(0.508 mm) and about 75 mils (1.905 mm), preferably about 50 mils (1.270mm). The SAC coating 16 is typically produced in one continuous sprayprocess. However, three separate spray events may be employed.

In a suitable setup for deposition of the above described layers, aplurality of bondcoated substrates 12 are loaded into a hollowcylindrical fixture such that the bondcoated surfaces face the innerdiameter of the cylindrical fixture. A plasma spray gun is positioned inthe interior of the cylindrical fixture for depositing the layers.

First, a base coat foundation layer 18 is applied to bond coat 14 to athickness of between about 5 mils (0.127 mm) and about 15 mils (0.381mm). Layer 18 is preferably a yttria partially stabilized zirconiaceramic layer (yttria partially stabilized zirconia herein refers to acomposition of about 12 weight percent or less yttria stabilizedzirconia). However, a composition of between about 6 weight percent andabout 20 weight percent yttria stabilized zirconia may be used, with apreferred range between about 7 weight percent and about 12 weightpercent yttria stabilized zirconia for material strength. Similarly,other zirconia based compositions, such as ceda stabilized zirconia,magnesia stabilized zirconia, calcia stabilized zirconia and mixturesthereof may be substituted for the yttria stabilized zirconia. A blendedlayer having a combination of about 7 weight percent yttria stabilizedzirconia and other yttria stabilized zirconia powders may also beemployed.

The particle size of the powder used for layer 18 (as well as the powderused for subsequently applied layer 20 and layer 22) may range fromabout 5 microns (0.005 mm) to about 175 microns (0.175 mm) with apreferred particle size of about 50 microns (0.050 mm) in mean diameter.The particles for layer 18, as well as for layers 20 and 22, areproduced from a spray dried and sintered process which results inspherical and hollow powders, as opposed to fused and crushed powderswhich are angular and solid. In general, the initial step of the spraydried and sintered process includes mixing raw zirconia and yttria to adesired weight percent ratio. This mixture is then combined with water(and conventional binders) to produce a slip. The slip is then fed intoa spray dryer which partially dries the slip by spraying the materialinto a heated chamber, thereby producing spherical and hollow powders.Then the material is heated at the sintering temperature in a furnacefor typically between about 4 hours and about 8 hours. This sinteringtemperature therein is usually about 60% to about 70% of the theoreticalmelting point of zirconium oxide.

Alternatively, a spray dried and plasma densifted process may be used,although this process may be more expensive than the above describedprocess. In general, the initial step of this process also includesmixing raw zirconia and yttria to a desired weight percent ratio. Thismixture is then also combined with water (and conventional binders) toproduce a slip. The slip is then fed into a spray dryer which partiallydries the slip by spraying the material into a heated chamber, therebyproducing spherical and hollow powder. Following the spray drying step,however, the powder is then fed through a plasma spray gun where theyttria and zirconia melt to produce a homogeneous composition.

The spherical and hollow morphology of the powder prior to deposition isa key factor for the success of the present invention, especially withrespect to its Superior abradability. For example, if solid particlesare present in the powder, more heat is required to melt the powder.This results in a dense coating which may not be very abradable. Also,the deposition efficiency for angular and solid particles is lessrelative to spherical and hollow particles. This is extremely importantfor manufacturing cost concerns.

Layer 18, the base coat foundation layer, is beneficial to the successof the segmented abradable coating system because it provides a toughceramic structure, starts segmentation of the deposited material intovertical microcracks, provides erosion protection and provides a thermalbarrier benefit. In addition, layer 18 bonds to the MCrAlY bond coat 14.

Layer 18 is typically plasma sprayed in air. It may be desirable to heatthe substrate 12 and monitor the temperature at less than about 600° F.(316° C.) to help segmentation of the material into verticalmicrocracks. This heating may be accomplished by application of heat tothe back side of the substrate 12 during material deposition. The abovedescribed heating parameters apply to layer 18, as well as to the othersubsequently applied layers. Preferably, however, the substrate 12 isnot heated except incidentally during spraying.

The processing parameters of the present invention are controlled toproduce vertical segmentation (approximately perpendicular to the bondcoat surface) and are specific to variables such as gun type and fixturegeometry. In general, we have found that a close gun to part spraydistance coupled with relatively high power deposition results indesirable vertical segmentation of between about 4 and about 8microcracks per inch. The parameters described herein were specificallytailored for use with a Sulzer Metco, Inc. 3MB air plasma spray gun anda cylindrical fixture having a 30 inch (0.76 m) diameter. One ofordinary skill in the art would appreciate that the parameters may varywith the use of a different spray gun and/or fixture. Accordingly, theparameters set forth herein may be used as a guide for selecting othersuitable parameters for different operating conditions.

Specifically, during the spray deposition of layer 18, the cylindricalfixture rotates at a speed between about 5 rpm and about 25 rpm, andpreferably at about 12 rpm. The plasma spray gun is located in theinterior of the hollow cylindrical fixture. The gun to part angle duringindividual part coating is between about 80 degrees and about 100degrees, and preferably about 90 degrees. The gun to part distance isvaried in increments from about a nominal 2 inches (0.05 m) (startingdistance) to about a nominal 5 inches (0.13 m) (end distance), andpreferably between about 2.75 inches (0.07 m) (starting distance) andabout 3.25 inches (0.083 m) (end distance) during production of layer18. This close gun distance is necessary for satisfactory verticalsegmentation. Gun traverse speed across each part during deposition isbetween about 1 in/min (0.03 m/min) and about 5 in/min (0.13 m/min),preferably about 4.4 in/min (0.11 m/min).

Powder feed rate is between about 15 grams/min and about 50 grams/min,and preferably about 35 grams/min. Carrier gas flow, such as nitrogen,is used to maintain the powder under pressure and facilitate powderfeed. The flow rate is between about 5 scfh (standard cubic feet/hour)(0.14 scmh (standard cubic meters/hour) and about 20 scfh (0.57 scmh),preferably about 11 scfh (0.31 scmh). Standard conditions are hereindefined as about room temperature (25° C.) and about one atmosphere ofpressure (101 kPa). Primary gas flow, such as nitrogen gas, in the gunis between about 80 scfh (2.27 scmh) and about 120 scfh (3.40 scmh), andpreferably about 99 scfh (2.80 scmh). Similarly, secondary gas flow,such as hydrogen, in the gun is between about 5 scfh (0.14 scmh) andabout 30 scfh (0.85 scmh), and preferably about 18 scfh (0.51 scmh). Gunvoltage is between about 60 volts and about 80 volts, and preferablyabout 75 volts. Similarly, gun amperage is between about 700 amps andabout 900 amps, and preferably about 736 amps. We have found the abovedescribed parameters to be optimum for the deposition process using theSulzer Metco 3MB plasma spray gun, but one skilled in the art wouldappreciate that the parameters are dependent on variables, including butnot limited to, powder type, powder size and especially type of gun.

Next a graded interlayer 20 is applied to base coat foundation layer 18to a thickness between about 3 mils (0.076 mm) and about 10 mils (0.254mm). This layer is also typically plasma sprayed in air. The compositionof the graded interlayer 20 is a blend of layer 18 (base coat foundationlayer) and layer 22, which is an abradable top coat subsequently appliedto layer 20. For ease of describing the composition of graded interlayer20, the composition of layer 22 will now be described. Layer 22 is anabradable top layer comprising zirconia. The nature of layer 22 is suchthat it must be soft enough to allow blade tips to cut into layer 22 andprovide sealing. The composition of layer 22 is typically a blend of 7weight percent yttria stabilized zirconia and 20 weight percent yttriastabilized zirconia. The ratio of the blend depends on the desiredcharacteristics of the resulting deposit. For example, if increasederosion resistance is desired, then an increase in the amount of 7weight percent yttria stabilized zirconia should be employed, whereas ifan increase in abradability is desired, then more 20 weight percentyttria stabilized zirconia should be added. However, in anotherembodiment essentially 100% of zirconia fully stabilized with yttria,such as 20 weight percent yttria stabilized zirconia, may be employedfor layer 22.

The composition of graded interlayer 20 is typically varied from astarting composition of a weight percent ratio of layer 18/layer 22 to afinal composition of a weight percent ratio of layer 18/layer 22. Forexample, we have employed a starting composition of 90/10 (weightpercent ratio of layer 18 to layer 22) to a final composition of 10/90(weight percent ratio of layer 18 to layer 22).

The deposition parameters for production of layer 20 are the same asthose previously described for production of layer 18, except that inthis case, preferably the gun to part distance is held constant at about3.25 inches (0.083 m). It may be possible to vary the gun to partdistance within the ranges described for production of layer 18, but wehave found that keeping the distance constant is optimum. The advantageof this graded interlayer is that it provides a strength link betweenlayer 18 and layer 22.

It should also be noted that the graded layer 20 may also be produced byother means, such as by application of individual layers of layer 18 andlayer 22, varied by pass.

After application of the graded layer 20, the above described abradablelayer 22 is sprayed on the graded interlayer 20 to a thickness betweenabout 15 mils (0.381 mm) and about 55 mils (1.397 mm). Preferably, thethickness of layer 22 is about 35 mils (0.889 mm). The depositionparameters for production of layer 22 are the same as those describedfor production of layer 20.

Porosity may be intentionally created within layer 22 by adding smallamounts of materials such as polyester or Lucite™ powder. The inclusionof about 1 to about 7 weight percent polyester powder (60 micron (0.060mm) nominal particle size) in layer 22 may produce a porosity on theorder of about 20-30 volume percent. High porosity levels, such aslevels greater than about 25 volume percent, may be unsatisfactorybecause of potential erosion of the coating. However, since a densestructure is desired for erosion resistance, addition of these materialsshould be minimized, if not eliminated entirely. It is desirous for theresultant density of layer 22 to be between about 90-95 percenttheoretical.

In an alternative embodiment of the present invention, layer 22 maycomprise alternating layers of a layer of 20 weight percent yttriastabilized zirconia and a layer of blended 7 weight percent yttriastabilized zirconia and 20 weight percent yttria stabilized zirconia(such as a 50-50 blend) for an overall thickness of preferably about 35mils (0.889 mm). The thickness of the layers may be between about 0.5mils (0.013 mm) and about 5 mils (0.127 nm) each. Each layer should beabout the same thickness.

In another embodiment of the invention, other materials, including butnot limited to ceria, magnesia, calcia or mixtures thereof may beemployed in place of yttria for the SAC system. However, yttriastabilized zirconia materials are recommended for SAC applicationsexceeding 1950° F. (1066° C.).

In another embodiment of the invention, alumina (99.0% purity) may beemployed. For example, a thin layer (less than about 5 mils (0.127 mm))of alumina may be sprayed on the bond coat 14 prior to application oflayer 18. Alternatively, a blended composition of alumina and less thanabout 12 weight percent yttria stabilized zirconia may be used for thebase coat foundation layer 18 (less than 10 weight percent aluminablend). The thin alumina layer may also be applied upon completion oflayer 18 and prior to application of layer 20.

In yet another embodiment of the invention, the SAC coating may consistessentially of layer 22. This coating would be desirable for militaryapplications. Due to the smaller size of military gas turbine enginecomponents, a thinner SAC coating is acceptable.

After application of the SAC coating, the flow path duct segment 10 isusually heat treated for stress relief. Specifically, the duct segment10 may be heat treated at about 1975° F. (1079° C.)+/-25° F. (14° C.),for about 4 hours and then force cooled to about 1100° F. (593° C.) inabout 22 minutes or less. The segment 10 is then force cooled to about1000° F. (538° C.) in about 7 minutes or less and again force cooled tobelow 300° F. (149° C.). This heat treatment preserves or even increasesthe useful life of the duct segment 10. For example, spallation of thecoating is reduced. This heat treatment may also be employed afterdeposition of layer 14 (bond coat) and prior to deposition of thefoundation layer 18, although it is not necessary for practice of thepresent invention.

The above described coating system is particularly suited to interactwith blade tips which are coated on their free end with an abrasivematerial, including but not limited to cubic boron nitride. Thisinteraction provides an effective sealing system.

The present invention will now be described by example which is meant tobe exemplary rather than limiting.

EXAMPLE

Nickel based high pressure turbine duct segments which were previouslycoated with a NiCoCrAlY oxidation resistant bondcoat were loaded into ahollow cylindrical fixture having a 30 inch (0.76 m) diameter such thatthe bond coated surface of the duct segments faced the center of thefixture. The HVOF process previously described was used to apply theNiCoCrAlY bondcoat to the duct segments.

A Sulzer Metco, Inc. 3MB air plasma spray gun was positioned in theinterior of the fixture which rotated at about 12 rpm. The gun waslocated at about a 90 degree angle to the inside surface of each ductsegment to be coated in turn. Three distinct layers were created on thebondcoated duct segments using the parameters set forth in Table 1below. The layers were deposited in one continuous spray process toavoid any potential weak links in the overall coating due to momentarilystopping the process. This continuous process was possible with the useof several Miller Thermal Model 1250 powder feeders, each containingpowder having a specific composition. The powders also had a sphericaland hollow morphology prior to deposition. The feeders can be computercontrolled to deposit the desired composition for each layer.Specifically, layer I, a base coat foundation layer, 0.010 inches (0.254mm) to 0.015 inches (0.381 mm) thick of between about 6 weight percentand about 8 weight percent yttrium oxide, remainder zirconium oxide(Sulzer Metco, Inc. 204NS powder) was deposited. Next, layer 2, a gradedinterlayer of varied composition from 90 weight percent layer 1/10weight percent layer 3 (starting composition) to 10 weight percent layer1/90 weight percent layer 3 (end composition) was deposited. The gradingwas performed in 8 uniform steps over a layer thickness of 0.005 inches(0.013 mm) at a constant gun distance of 3.25 inches (0.083 m), as notedin Table 1 below. Layer 3, an abradable top layer, was then deposited toa thickness between about 0.025 inches (0.635 mm) and 0.040 inches(1.016 mm). The composition of layer 3 was a 50--50 blend of 1) thecomposition of layer 1 and 2) between about 18.5 weight percent andabout 21.50 weight percent yttrium oxide, remainder zirconium oxide(Sulzer Metco, Inc. 202 powder or equivalent). A multi-layer segmentedabradable coating having between about 4 and about 8 verticalmicrocracks per inch resulted.

                  TABLE I                                                         ______________________________________                                                        Layer 1                                                                              Layer 2  Layer 3                                       ______________________________________                                        fixture rotation speed (rpm)                                                                    12       12       12                                        gun traverse speed across ca. scal                                                              4.4      4.4      4.4                                       (in/min)                                                                      gun to scal distance (in)                                                     start             2.75     3.25     3.25                                      end               3.25     3.25     3.25                                      gun to seal angle (degrees)                                                                     90       90       90                                        gun voltage (volts)                                                                             75       75       75                                        primary gas flow, N.sub.2 (scfh)                                                                99       99       99                                        secondary gas flow, H.sub.2 (scfh)                                                              18       18       18                                        gun amperage (amps)                                                                             736      736      736                                       carrier gas flow, N.sub.2 (scfh)                                                                11       11       11                                        powder feed rate (grams/min)                                                                    35       35       35                                        ______________________________________                                    

An advantage of the present invention is its superior abradability ascompared to a segmented coating having a composition of 7 weight percentyttria, balance substantially zirconia (7 YSZ). This superiorabradability of the present invention results, in part, because 7 YSZcontains approximately equivalent proportions of tetragonal ZrO₂ andcubic ZrO₂. Tetragonal ZrO₂ is stronger than cubic ZrO₂ due to itslattice structure. Thus, 7 YSZ is a strong, effective erosion resistantmaterial. However, 20 weight percent yttria, balance substantiallyzirconia (20 YSZ) does not contain tetragonal ZrO₂ (only cubic ZrO₂),and thus is more abradable (i.e. sorer). As stated previously, 20 YSZ isa desirable composition for the abradable top layer 22. In a ductsegment application, effective abradability is an essential requirementfor efficient engine operation. Different material combinations can beselected for specific engine models having different needs. For example,a SAC coating with less abradability and more erosion resistance can beselected, if desired. In such a case, SAC coating may comprise a blendof 7 YSZ and 20 YSZ as the abradable layer (layer 22). The blend wouldprovide the needed erosion resistance (from the 7 YSZ) withoutsacrificing abradability. An important point to remember is that thesegmented abradable coating of the present invention, with its uniquestructure and variety of material combinations, can be tailored forengine specific duct segment applications. The three layer approachprovides a means of tailoring the long-term thermal insulation benefitprovided by the initial layers (layer 18 and layer 20) and theabradability benefit provided by the top layer (layer 22) in a givenengine application.

Although the invention has been shown and described with respect todetailed embodiments thereof, it should be understood by those skilledin the art that various changes in form and detail may be made withoutdeparting from the spirit and scope of the invention. Specifically,although the present invention has been described as a segmentedabradable ceramic coating for aircraft gas turbine engine duct segments,the present invention may have other potential applications, includingbut not limited to, a thermal barrier coating on gas turbine enginecomponents, such as vanes, and as a segmented abradable ceramic coatingsystem for land based turbine applications. The present invention mayalso have application in the automotive industry as a coating forautomotive engine components, such as pistons.

What is claimed is:
 1. A method of producing a segmented abradableceramic coating system including a base coat foundation layer, a gradedinterlayer, and an abradable top layer, said method comprising:applyingthe base coat foundation layer on a MCrAlY bondcoated metallic substrateusing a spray gun, said base coat foundation layer comprising a layer ofmaterial selected from the group consisting of zirconia stabilized withceria, zirconia stabilized with magnesia, zirconia stabilized withcalcia, zirconia stabilized with yttria, and mixtures thereof, whereinthe distance between the gun and surface to be coated is varied inincrements from a starting distance of about 2 inches to an end distanceof about 5 inches during deposition of the base coat foundation layer toproduce the base coat foundation layer having a thickness of betweenabout 5 mils and about 15 mils; applying the graded interlayer on thebase coat foundation layer using a spray gun, said graded interlayercomprising a compositional blend of the base coat foundation layer andthe abradable top layer; wherein the distance between the gun andsurface to be coated is held constant during deposition of the graded.interlayer to produce the graded interlayer having a thickness ofbetween about 3 mils and about 10 mils; and applying the abradable toplayer on the graded interlayer using a spray gun, said abradable toplayer comprising zirconia, wherein the distance between the gun andsurface to be coated is held constant during deposition of the abradabletop layer, wherein each layer comprises vertical segmentation, as wellas powder particles which are spherical and hollow, prior to deposition,for increased abradability.
 2. The method of claim 1 wherein thedistance between the gun and the surface to be coated is held constantat about 3.25 inches during deposition of the graded interlayer anddeposition of the abradable top layer.
 3. The method of claim 1 whereinthe segmented abradable ceramic coating system includes between about 4and about 8 vertical microcracks per inch.
 4. A method of producing asegmented abradable ceramic coating system including a base coatfoundation layer, a graded interlayer, and an abradable top layer, saidmethod consisting essentially of:applying the base coat foundation layeron a MCrAlY bondcoated metallic substrate using a spray gun, said basecoat foundation layer consisting essentially of between about 7 weightpercent and about 12 weight percent yttria stabilized zirconia, whereinthe distance between the gun and surface to be coated is varied inincrements from a starting distance of about 2 inches to an end distanceof about 5 inches during deposition of the base coat foundation layer toproduce the base coat foundation layer having a thickness of betweenabout 5 mils and about 15 mils; applying the graded interlayer on thebase coat foundation layer using a spray gun, said graded interlayerconsisting essentially of a compositional blend of the base coatfoundation layer and the abradable top layer; wherein the distancebetween the gun and surface to be coated is held constant duringdeposition of the graded interlayer, said graded interlayer having astarting deposited composition of substantially the composition of thebase coat foundation layer and is continuously graded to an endcomposition of substantially the composition of the abradable top layer,said graded interlayer having a thickness of between about 3 mils andabout 10 mils; and applying the abradable top layer on the gradedinterlayer using a spray gun, said abradable top layer consistingessentially of a blend of 7 weight percent yttria stabilized zirconiaand 20 weight percent yttria stabilized zirconia and is sufficientlysoft to allow blade tips to cut into the abradable top layer and providesealing, wherein the distance between the gun and surface to be coatedis held constant during deposition of the abradable top layer, saidceramic coating system further comprising vertical segmentation andpowder particles which are spherical and hollow prior to deposition.